WASHINGTON — As the Army staff sergeant suspected of killing 16 civilians was flown out of Afghanistan, two military officials told McClatchy on Wednesday that investigators combing his medical records had found "no smoking gun" to explain the rampage.

The officials said that the suspect — a 38-year-old father who survived three tours in Iraq before deploying to Afghanistan in December — had no evidence of a serious traumatic brain injury or of post-traumatic stress, despite widespread speculation that those conditions were factors in the killing spree.

The officials, who spoke on condition of anonymity because the investigation was ongoing, called the suspect's 10-year Army career "unremarkable" and said he hadn't had reports of discipline problems while in Afghanistan.

The lack of a clear cause and effect had Pentagon officials admitting they're befuddled by what caused the attack. The suspect was apprehended as he returned to the base. He acknowledged the killings, then asked for an attorney within minutes of being captured and isn't cooperating with the investigation, officials said.

The sergeant is suspected of having left his outpost in Kandahar province about 1 a.m. Sunday, walked a mile, entered three homes and fired upon unarmed villagers, killing 16 including nine children, and wounding several others, some critically, according to Afghan officials. American officials haven't confirmed the number of casualties.

Military investigators arrived at the crime scenes hours after the shooting and collected shell casings and other evidence. But many of the victims already had been buried in accordance with Islamic customs, the officials said.

Investigators are pursuing a variety of explanations for the killings. Alcohol was found at his small outpost in Kandahar's Panjway district, but officials haven't determined whether alcohol or drugs contributed to the attack.

They are also probing whether an email that the sergeant reportedly received from his wife about their failing marriage could have prompted the incident.

Navy Capt. John Kirby, a Pentagon spokesman, said the suspect was flown to an undisclosed U.S. military facility in another country because there was no long-term facility to hold him in Afghanistan. It wasn't clear how the news would be greeted by Afghans, many of whom have called for him to be tried in Afghanistan. Charging the suspect outside of Afghanistan also could mean that not all Afghan witnesses will appear at his trial.

Media reports have focused on the fact that this was the suspect's fourth deployment to a war zone, and that he had been in a non-combat-related vehicle accident in Iraq in 2010, which left him with a mild case of traumatic brain injury.

Dr. Elspeth Cameron Ritchie, formerly a behavioral health director in the surgeon general's office, didn't know the intimate details of the case but speculated that the suspect's actions were inconsistent with post-traumatic stress disorder or traumatic brain injury.

"His reported behavior was more in line with a psychotic episode," Ritchie said.

Military statistics show that about 92,000 troops have served at least four tours in Iraq or Afghanistan, and tens of thousands have served more. In addition, almost 70,000 troops have served with PTSD and almost 180,000 have suffered a traumatic brain injury.

Twenty-seven percent of troops screened positive for mental health problems following their third deployment, according to the Iraq and Afghanistan Veterans of America, an advocacy group.

The suspect is known to have undergone standard psychological testing after each tour. In addition, he underwent several medical evaluations after the 2010 accident in Iraq.

Matt Gallagher, a senior fellow at the Iraq and Afghanistan Veterans of America, who served 15 months in Iraq, said that the line between experience and burnout from multiple tours is difficult to draw. Many veterans of multiple tours blur the distinction between combat and civilian life, and experts say the stress can be enormous — especially in wars in which creating goodwill among the population is seen as fundamental to success, meaning every interaction with locals has strategic importance.

"But we don't know what happened here, and far and away most people serve honorably and go home," he said.

Until he is charged, the military is not expected to release the suspect's name, a process that in complicated cases can take weeks. He was a sniper in the 3rd Infantry Regiment of the 3rd Stryker Brigade, 2nd Infantry Division, based at Joint Base Lewis-McChord in Washington state.

Pentagon officials have suggested he will be charged with premeditated murder, which could carry the death penalty.

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